1. Field of the Invention
This invention relates generally to optical scanners and, more particularly to devices and methods useful in scanning storage phosphor imaging plates.
2. Description of the Background Art
Phosphors emit light energy upon exposure to other radiant energy (phosphorescence). Phosphors absorb radiant energy which excites electrons in the phosphor into higher energy states. The higher energy states are unstable and the phosphor electrons fall back into a lower energy state emitting the energy differential as light. Typically, the emitted light energy has a different wavelength than the radiant energy inducing the phosphorescence.
Phosphors have many uses, e.g., television picture cathode ray tubes. Storage phosphors, however, have distinct properties that make them highly desirable as detectors of light and other forms of radiation. Storage phosphors remain in an excited state for very long time scales. While phosphors generally relax to their ground state after excitation in a matter of a few thousandths or hundredths of a second, storage phosphors retain an excited state for days or even weeks. This property is referred to in this application as retention. For example, storage phosphors have been patented and produced that have a retention of up to 75% in a 24 hour period when exposed to samples labeled with .sup.32 P, a radioactive isotope.
Storage phosphors retain a latent image when exposed to a two dimensional pattern of radiation, analogous to film. Storage phosphors, however, do not require development. In storage phosphors the latent image is obtained by reading out the storage phosphor with a scanning beam of light. The read out beam produces an emission from the storage phosphor The color of the emission is different from the readout color. The intensity of the emission is proportional to the original degree of localized radiation exposure retained by the storage phosphor.
Storage phosphors have a very high quantum efficiency when exposed to some types of radiation and so are desirable as a storage medium. Storage phosphors have been proposed as detectors of radioactively labeled biological samples. Storage phosphors have also been proposed as optical information storage devices and optical matrix processors.
Different devices for scanning reflected and phosphorescent light have been described. Simpkins, U.S. Pat. No. 3,588,514, discloses a reflected light facsimile scanner utilizing optical fiber bundles containing emitting and collecting fibers.
Ogland et al., U.S. Pat. No. 3,746,840, discloses a device for high resolution readout of information stored on a film. The device comprises a slit equal in width to the desired resolution with optical fibers behind the slit of a diameter equal to the slit width. As light crosses the slit, the optical fibers collect the light and transmit it to detectors.
Wilde et al., U.S. Pat. No. 3,836,225, discloses fiber optic laser scanners which use two optical fiber sets attached to electromagnetic magnetic coils which can deflect the beam in perpendicular dimensions.
Duguay, U.S. Pat. No. 3,892,468, discloses a passive array of variable length optical fibers which function as a dynamic scanner. Each consecutive fiber is incrementally longer than the preceding fiber so that light entering the fibers at the same time exits the fibers at different times and can be correlated with a different location.
Balliet et al., U.S. Pat. No. 4,467,196, discloses a manually controlled bar code scanner probe. The scanner includes a single optical fiber which alternately emits and collects light. The probe contains a half mirror allowing both emission and collection of light in the single optical fiber.
Waszkiewicz, U.S. Pat. No. 4,481,142, discloses a method to control scanner resolution by reading variable numbers of sensors in a sensor array.
Moriguchi, U.S. Pat. No. 4,490,740, discloses a multicolor optical reading device which comprises a high intensity light and several filters of different hues. The light of different hues is transported to the reading plate by optical fibers and is reflected to mirrors and a detector.
Margolin, U.S. Pat. No. 4,748,680, discloses a color scanning device utilizing a plurality of fiber optic arrays similar to Waszkiewicz in which each array corresponds to a different color associated with a color filter.
Tomei et al., U.S. Pat. No. 4,877,966, discloses a device for measurement of low-level laser induced phosphorescence. The laser is directed through a beam expander and then aimed by mirrors. The induced phosphorescence is collected by a fiber optic face plate and passed to a photomultiplier tube.
The scanners described above do not take full advantage of the potential of storage phosphors. Storage phosphors have an inherently high capacity for storing incident radiation in a latent image. Storage phosphors have an inherent dynamic range on the order of 10.sup.5 and higher. Scanners of storage phosphors need to have the highest possible collection efficiency of the light emitted by the storage phosphors. The lower the smallest unit of light that can be discerned by the scanner, the greater the dynamic range of the scanner.
Further, scanners which emit radiation or collect phosphorescence from a relatively large area of the plate lose specificity. Storage phosphor scanners require high resolution or a very small spot size of the read out beam.
Also, the scanners must be capable of differentiating between reflected radiant energy and phosphor emissions. Often, more reflected radiant energy than phosphorescence strikes scanner collecting mechanisms. In order to accurately read the information stored on the plate, the scanner must only sense the phosphorescence.
Storage phosphors can resolve features in the original sample as small as one tenth of a millimeter. In some cases, it may be desirable to be able to read out the latent image at a low resolution to minimize scan time and to minimize the amount of memory required to store an image. It may then be desirable to go back and scan sub-regions or features of the latent image in the storage phosphor that require higher resolution. This requires a scanner with not only high resolution and specificity, but also repeatable alignment of a low resolution scan with a subsequent high resolution scan. This requires a scanner with an addressable scanning head design. This would also be required for scanners used to retrieve information from storage phosphors used as optical memory devices.
For these reasons it is desirable to provide scanning devices capable of exciting phosphorescence in a localized region of a storage phosphor imaging plate and efficiently collecting and sensing low level phosphorescence. The present invention provides a system for precise and efficient excitation and collection of phosphorescence.